PyChecker A tool for finding common bugs in Python source code. It finds problems that are typically caught by a compiler for less dynamic languages, like C and C++. [Open Source, BSD-like]

PyLint An extensible tool that checks if a module satisfies a given coding standard.

PyUnit An extensible unit testing framework for Python, based on Java's de facto standard JUnit framework.

twill A tool for Web application testing. It implements a simple scripting language that can be used interactively or from scripts; this language contains commands for browsing the Web, filling out forms, and asserting various conditions.